It was recently established that crab d(A-T) polymer contains 1 to 3% of covalently bound RNA dispersed throughout the DNA chain. About 2/3 of this RNA occurs as a single monoribonucleotide (...d-d-r-d-d...); about 1/3 occurs in a sequence of two or more r's, because ribomononucleotides are detectable after the alkaline hydrolysis. Three mixed dinucleotides (dC-rG, dT-rA, and dT-rG) were isolated from the exhaustive digest of crab d(A-T) polymer with pancreatic DNase I, suggesting that this enzyme is capable of hydrolyzing the r-d linkage as shown ...dN alpha-rN beta-dN gamma-dN delta... . It is intended to expand this finding to other DNAs commonly used in the laboratories, to verify the observation on the sugar specificity of DNase I, and to compare different single-strand specific nuclease with respect to their ability to liberate derivatives of ribose.